Oxidants are used in a wide variety of water treatment applications to reduce chemical oxygen demand (COD), increase cleanliness, and/or provide biological control. Such treatment programs are especially useful in papermaking, pulp production, waste water treatment, recreational waters, and recirculating cooling waters. While sodium hypochlorite or chlorine gas can be used as microbicides, the addition of unstabilized free halogen oxidants suffers from inefficiencies generated by various system components or impurities, resulting in an oxidant demand greater than that required for microbial control in the absence of such components. This is due, in part, to the high oxidation potential of free halogens and efficient kinetic pathways for their degradation, resulting in unselective oxidation of various organic and inorganic species and leading to inefficient utilization and the generation of undesirable byproducts. In this regard, the use of organic halogen stabilizers is known to reduce unwanted side reactions and decomposition of free halogens in the presence of sunlight, process additives, paper making furnish components, and other system components, while still providing the desired microbicidal performance.
U.S. Pat. No. 7,407,590 teaches the use of hypochlorite stabilization chemistry using hydantoins for controlling sessile bacterial growth (i.e., biofilm formation) in aquatic systems. The inventors found the method to be more efficacious against sessile bacteria than the free halogen treatments of the prior art. In addition, the method of that invention required less total halogen to achieve biofilm control than other methods involving free halogen treatments. The inventors further found that it is desirable to utilize the most efficient oxidizing halogen programs because oxidizing halogens generate absorbable organic halogen (AOX) in side reactions with organic matter. Limiting the formation of AOX reduces the environmental impact of the treatment.
Another example of a halogen stabilizing technology is described in U.S. Pat. No. 5,565,109, which teaches that selected N-hydrogen compounds, such as 5,5-dimethylhydantoin (DMH), dramatically improve the bactericidal efficacy of hypochlorite solutions in pulp slurries, while significantly reducing the amount of hypochlorite required to achieve biological control. This efficacy enhancement is believed to result from the conversion of free halogen to combined halogen by the N-hydrogen compound, such as DMH, which effectively increases the lifetime of active halogen and its persistence in the presence of organic components and other contaminants. Due to this stabilizing effect, at any given time the residual halogen concentration of a system in a papermaking application is greater than the residual free chlorine when using hypochlorite alone. Another salient feature described was the use of a wide range of molar ratios of hypochlorite to stabilizer compound, ranging from about 0.1:1 to about 10:1. Although the patent teaches the improved stabilization of free chlorine sources, such as sodium hypochlorite, by N-hydrogen compounds in the presence of material exhibiting a free chlorine demand, the patent does not concern itself with synergistic benefits of N-hydrogen compound mixtures upon total chlorine yields for inorganic N-hydrogen compounds, such as ammonium salts, at high Cl2:N molar ratios (e.g, Cl2:NH3>2-3; vide infra).
In addition to the examples above, inorganic N-hydrogen compounds, such as ammonia and its corresponding conjugate acid salts, as well as urea are used for halogen stabilization and microbial control. Ammonia can react with chlorine or hypochlorous acid/hypochlorite systems to form mono-, di-, and trichloramines, depending upon process conditions such as pH. Not to be limited by theory, the latter two forms, dichloramine and trichloramine, can form when chlorine/hypochlorite:N molar ratios are greater than 1:1 and are particularly unstable, decomposing exothermically and fairly rapidly to nitrogen gas and hydrogen chloride in aqueous systems. Depending upon specific system factors, such as alkalinity, this can lead to a dramatic decrease in pH and increased corrosion in some systems. In fact, this instability has been used as a means for ammonia removal in the potable water industry, a process well-known to those skilled in the art as “breakthrough” chlorination.
Similarly, decomposition of urea is usually observed in the presence of excess hypochlorite (i.e., when the NaOCl:urea molar ratio is greater than 2 or, since urea has two nitrogen atoms, the NaOCl:N equivalent ratio is greater than 1). For instance, U.S. Pat. No. 4,508,697 teaches a method whereby the residual oxidant content in a waste hypochlorite process stream is destroyed via the use of urea, the major reaction in the process being:3NaOCl+CO(NH2)2→3NaCl+N2+CO2+H2O
Ultimately, the process described reduces to biological oxygen demand (B.O.D.) of the effluent in order to meet federal EPA guidelines.
Nonetheless, stabilized halogens in the form of monohalamines, such as mono-chloramine, are also known to be effective for both planktonic and sessile bacterial control in a variety of industrial applications, such as pulp and paper applications. According to the prior art, careful control of conditions for mixing the halogen source with the ammonia or ammonium salt source is required. For instance, U.S. Pat. No. 6,132,628 describes a complex system for the careful formation of halamines for treating aqueous systems to inhibit bacterial growth. Key features of the described process method include the dilution of the ammonium salt source to a preferred concentration of 0.1-6.0%, maintaining an oxidant:N molar ratio of less than or equal to 1, although preferably about 1, and maintaining the pH of the biocidal mixture to at least 9.0.
Similarly, patent application WO 2007/089539 A2 and U.S. Patent Application Publication No. 2007/0178173 A1 describe a chemical composition for microbial control in aqueous systems whereby a free chlorine source is combined with urea in a specifically defined molar ratio of chlorine (as Cl2) to urea in the range of 2:1 to 1:2 and an alkali base in order to maintain a pH greater than 10.0. According to the inventors, the presence of the alkali base provided both greater stability as well as improved biocidal efficacy for the given compositional range. However, even at high pH (12.4-13.4) and Cl2:urea ratios of 1:1-2:1, the total halogen yield is only 48-69%.
U.S. Patent Application Publication No. 2003/0029812 A1 teaches a method for controlling the growth of microorganisms or killing microorganisms in aqueous solution via the combination of a free halogen-generating biocide, an N-hydrogen compound stabilizer, and a quaternary ammonium compound or biocidal amine. However, compositions for reducing the impact of vapor phase corrosion and increasing the stability of halogenated inorganic N-hydrogen compounds are not disclosed.
Hence, an important limitation of the use of the ammonia, ammonium salt, and urea systems described above is the strict requirement to maintain specific maximum molar ratios of Cl2:NH3 (1:1 max) or Cl2:urea (2:1 max; corresponding to a 1:1 ratio of Cl2:N). In view of the above, it is an object of the present invention to provide a composition for stabilizing sources of active halogen in aqueous systems wherein a wide range of Cl2:N ratios can be used while maintaining a high bactericidal efficacy, low formation of absorbable organic halogen due to limited reactivity with suspended or dissolved organic impurities, and negligible vapor phase corrosivity.